There are heretofore known power shovels provided with a plurality of hydraulic actuators such as a boom cylinder, an arm cylinder, a bucket cylinder, a running motor and a turning motor, etc., and hydraulic circuit for supplying fluid under pressure into these hydraulic actuators, which includes a plurality of valves connected to discharge passages of a hydraulic pump, the arrangement being made such that fluid under pressure may be supplied by changing over each of the valves into each of the hydraulic actuators.
However, when the plurality of valves are operated simultaneously, fluid under pressure is supplied into hydraulic actuators whose load pressures are low, whilst fluid under pressure is not supplied into hydraulic actuators whose load pressures are high.
To eliminate such difficulties, there is provided a hydraulic circuit provided with a plurality of pressure compensating hydraulic valves which can be operated at the same time so as to supply or distribute fluid under pressure into each hydraulic actuator at a uniform flow rate, as disclosed in Japanese Laid-Open Patent Application NO. SHO 60-188604.
The pressure compensating type hydraulic valve disclosed in the above-mentioned Japanese Patent Applications specification is constructed such that, as shown in FIG. 6, a spool 2 is slidably mounted in a valve body 1 so as to allow inlet ports 3 and 4 to be selectively connected with and disconnected from actuating ports 5 and 6, respectively, and a bridging passage 7 formed in the valve body 1 is arranged to be selectively connected with and disconnected from the actuating ports 5 and 6, respectively, and provided with a pressure compensating type hydraulic valve adapted to be set by a maximum working pressure in a plurality of hydraulic actuators, the arrangement being made such that fluid under pressure may be supplied into the actuating ports 5, 6 in the plurality of pressure compensating type hydraulic valves thereby ensuring supply of fluid under pressure into each hydraulic actuator at a uniform flow rate even in case a plurality of hydraulic actuators are operated simultaneously and their load pressures are different.
In such a pressure compensating type hydraulic valve, since the pressure in the first and second actuating ports 5 and 6 is set by one and the same pressure compensating valve, the pressure compensation characteristic obtainable when fluid pressure is supplied into the first actuating port 5 becomes the same as that obtainable when fluid pressure is supplied into the second actuating port 6. As a result, in case the spool 2 is moved by the same stroke in opposite directions, fluid under pressure is supplied at the same flow rate into either one of the actuating ports 5 and 6. Therefore, to supply fluid under pressure into the ports 5 and 6, respectively, at different flow rates, the area of opening in each port must be decided by regulating the stroke of the spool 2 depending on the flow rate of fluid required by the hydraulic actuator, and so the operation itself becomes troublesome. Thus, in case a large number of hydraulic actuators are operated simultaneously, regulation needs to be made at a position where a big change occurs in the maximum area of opening of the spool.
Further, since the load pressure is introduced into the pressure compensating type hydraulic valve by allowing the first and second actuating ports 5 and 6 to be connected with or disconnected from the bridging passage 7 by land portions 2a, 2a of the spool 2 when the spool is held at its neutral position, there is a tendency of fluid under pressure leaking from either the actuating port 5 or the actuating port 6 through either one of the land portions 2a, 2a and the bridging passage 7 into the fluid reservoir or tank, thus rendering it impossible to stop the movable component of the hydraulic actuator against the load pressure.